Background:Anxiety is one of the main problems in patients waiting for diagnostic procedures for digestive disorders, which unfavourably affects their recovery process. Therefore, it is essential to modify or eliminate it by approved methods that are not associated with major adverse effects. This study aimed to determine the effect of peppermint aromatherapy on the level of anxiety in patients undergoing colonoscopy.Methods:This clinical trial was conducted on 50 patients undergoing colonoscopy referred to one of the training Quaem Hospital affiliated to Mashhad University of Medical Sciences, Mashhad, Iran, in 2015. The samples were selected through convenience sampling method, and they were randomly assigned into two groups of intervention and control. The subjects in the intervention group inhaled three drops of pure peppermint essential oil and 5 cc distilled water with a nebulizer mask for 10 min, half an hour prior to the colonoscopy. However, the patients in the control group inhaled distilled water. The level of anxiety was estimated in the pre- and post-intervention phases using Spielberger State-Trait Anxiety Inventory. Data analysis was performed in SPSS software, version 19 using Fisher’s exact and Chi-squared tests, as well as paired and independent samples t-tests.Results: According to the results of this study, the mean scores of anxiety were changed from 52.1±6.9 and 52.4±7.2 at the pre-intervention phase to 45.5±4.0 and 50.1±5.9 at the post-intervention stage, respectively (P<0.009).Conclusion:Regarding the results, peppermint aromatherapy can decrease the level of anxiety before undergoing a colonoscopy. Therefore, it is recommended to employ this non-pharmacological, low-risk, and pleasant method for patients before invasive procedures such as colonoscopy.